You have borders because you want the image to be inside of the title safe area and then the layout is constrained because the chip is designed to show a 40x25

You have borders because you want the image to be inside of the title safe area and then the layout is constrained because the chip is designed to show a 40x25 character area of characters 8 pixels wide, and outputting 8 pixels takes exactly as much time as one clockcycle of the cpu, they execute in lockstep. And of course you can’t just zoom in a little because a character is 8x8 pixels and the video signal has a fixed number of lines on the screen so 25 characters of 8 lines tall is just a fixed part of the screen (which is 312 lines tall on PAL). In hindsight it is a bit silly that there is no easy way to display sprites over the borders though, to do that you have to wait until the video chip is displaying the last character and then configure the screen a bit less wide/tall so it oversteps the end and never stops outputting.